The present invention relates to novel photopolymerizable acrylic varnish compositions.
The invention is especially suitable for varnishes for coating substrates comprising polycarbonate or polymethyl methacrylate (PMMA).
It is known that the weathering stability of paints and varnishes may be improved by using mixtures of UV absorbers of the benzotriazole type and what are known as HALS (Hindered Amine Light Stabilizer) compounds, which are light stabilizers of the sterically hindered amine type.
As a benzotriazole, mention may be made of 2-(2xe2x80x2-hydroxy-5xe2x80x2-methylphenyl)benzotriazole with the formula 
the CAS number 2440-22-4, and a molecular weight of 225.
As HALS compound use is very frequently made of HALS compounds of type 1, such as bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate with the formula 
the CAS number 52829-07-9, and a molecular weight of 481,
or HALS compounds of type 2, such as, in particular, bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate with the formula 
the CAS number 41556-26-7, and a molecular weight of 509.
The stabilization of photopolymerizable acrylic varnishes is known to be generally more problematic. In acrylic varnish formulation technology it is in fact well known that a certain number of stabilizers which are intended to restrict the color variations and the loss of transparency of the varnishes must be incorporated into these varnishes. Certain known stabilizers, however, may adversely affect the adhesion properties of the varnish on the substrate.
For instance, the use of benzotriazole derivatives is accompanied by an inhibition of polymerization both on the surface and in the core of the product. Furthermore, the basicity of the HALS compounds of type 1 leads, additionally, to detachment phenomena in the course of resistance testing under wet heat conditions.
Moreover, the choice of stabilizers also depends to a large extent on the hardness of the acrylic varnish.
For instance, a general distinction is made between the following:
xe2x80x9cHardxe2x80x9d varnishes having a Persoz hardness in accordance with NFT standard 30016 of more than 300 seconds, which were the first varnishes to be used at all. These varnishes may comprise two-component varnishes, which are generally heat-cured, or xe2x80x9cUV varnishesxe2x80x9d, which are cured under UV radiation. These varnishes are generally applied in a thin film, whose thickness is generally from 5 to 15 xcexcm.
xe2x80x9cElasticxe2x80x9d or xe2x80x9cflexiblexe2x80x9d varnishes having a Persoz hardness of less than 120 seconds, which are in turn subdivided into two classes:
two-component varnishes of the crosslinked polyurethane (PU) type, which are heat-cured at temperatures between 120 and 130xc2x0 C. over a period of about 40 to 60 minutes, and
xe2x80x9cUVxe2x80x9d varnishes, which are cured under ultraviolet radiation.
These elastic or flexible varnishes belong to the class of the xe2x80x9cself-healingxe2x80x9d varnishes.
In the particular case of the application of the acrylic varnishes to polycarbonate substrates, especially in the case of varnishes for vehicle headlamps, it must be borne in mind when formulating the varnishes that the polycarbonate, owing to its structure, is highly susceptible to yellowing on weathering. It is therefore necessary to add products to the varnish composition which stabilize the varnish itself to yellowing and damage by cracking as a consequence of depolymerization of the polymer chains. The addition of UVA and UVB filters to the varnish is therefore obligatory. Their concentration in the varnishes is restricted, however, by the fact that the absorbers possess an intrinsic color and, owing to their sluggish reactivity, act as plasticizers, leading to bleeding phenomena or to a reduction in the chemical resistance of the varnish. On bleeding, moreover, transparency is lost. At a given concentration, therefore, the protection of the polycarbonate depends heavily on the remaining thickness. This relationship obeys the Lambert-Beer law. This means, in other words, that, applied at a low thickness, the UV filter concentration must be increased very rapidly, which adversely affects the performance properties of the varnish and makes it unsuitable for use for UV polymerization.
In the international patent application WO 96/23836 it has already been proposed to use mixtures of triazines and neutral HALS compounds of type 2 for photopolymerizable elastic varnishes which are intended in particular for protecting glazing made from synthetic plastic material. However, these same UV absorber systems are unsuitable for varnish formulations of the urethane acrylate type for the preparation of hard varnishes, especially varnishes having a Persoz hardness of more than 300 seconds.
It is found that, when attempts are made to use the UV absorber systems developed in the field of the abovementioned elastic varnishes for hard varnishes, it is impossible to achieve acceptable adhesion conditions without pretreating the substrate or using a primer which promotes the adhesion of the varnish to the substrate.
In the course of our own investigations it has now been found that, by replacing the HALS compounds by antioxidants from the class of the sterically hindered phenols, it is possible to obtain an improvement in the optical properties without any deterioration in the varnish adhesion properties observed with the presence of HALS compounds in the case of hard varnish compositions.
By hindered phenols are meant, here, a well-known class of phenol derivatives which are widely employed as antioxidants.
The derivatives of the sterically hindered phenol type used in accordance with the invention comprise oligomerized, dimerized or substituted derivatives of methyl-2,6-di-tert-butyl-4-methylphenol. The substituents are generally selected from alkyl groups having preferably 6 to 18 carbon atoms, isocyanurate, triazine, thioalkyl, amino or cycloamino.
The 4-methylphenol may be dimerized or oligomerized with itself or with cyclopentadiene derivatives and also with various substituents, as defined in particular in xe2x80x9cPlastics Additives Handbookxe2x80x9d, R. Gxc3xa4tcher and H. Mxc3xcller, edition 3/1990, page 40-50.
These products are commercially available as stabilizers for thermoplastic polyolefins, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene, poly-4-methylpentene and polybutylene to counter oxidation and degradation of these polymers, and they serve primarily as stabilizers during the processing of these polymers and subsequently as stabilizers for preventing depolymerization.
As already mentioned above, the present invention is based on the surprising discovery that these particular antioxidants, which have never before been used in acrylic varnish formulations, may be used in acrylic varnish formulations and, in particular, in formulations for hard acrylic varnishes, especially acrylic varnishes for protecting polycarbonate or PMMA surfaces, without these formulations being destabilized during photochemical polymerization, especially during polymerization under UV radiation, and without the varnish adhesion properties being adversely affected during the aging of this varnish under wet heat conditions.
In accordance with one of its substantial features, the invention accordingly provides a photopolymerizable unsaturated acrylic varnish composition which comprises an antioxidant from the class of the sterically hindered phenols.
The sterically hindered phenol is selected advantageously from products having a molecular mass of more than 300.
As examples of products especially suitable for the implementation of the invention, mention may be made of the following products, which are listed below together with their molecular masses and their Chemical Abstracts number.
2,6-Di-tert-butyl-4-nonylphenol
M=333
CAS=4306-88-1
2,2xe2x80x2-Methylenebis(4-methyl-6-tert-butyl-phenol)
M=341
CAS=119-47-1
4,4xe2x80x2-Butylidenebis(2-tert-butyl-5-methyl-phenol)
M=383
CAS=85-60-9
4,4xe2x80x2-Thiobis(2-tert-butyl-5-methylphenol)
M=358
CAS=96-69-5
2,2xe2x80x2-Thiobis(6-tert-butyl-4-methylphenol)
M=358
CAS=90-66-4
Octadecyl 3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxy-phenyl)propionate
M=531
CAS=2082-79-3
Tetrakismethylene(3,5-di-t-butyl-4-hydroxy-hydrocinnamate)methane
M=1178
CAS=6683-19-8
Tris(3,5-di-t-butyl-4-hydroxybenzyl)-isocyanurate
M=784
CAS=27676-62-6
2,2xe2x80x2-Thiodiethylbis(3,5-di-tert-butyl-4-hydroxyphenyl)propionate
M=642
CAS=41484-35-9
1,1,3-Tri(2xe2x80x2-methyl-4xe2x80x2-hydroxy-5xe2x80x2-tert-butylphenyl)butane
M=545
CAS=1843-03-4
2,2xe2x80x2-Methylenebis-6-(1-methylcyclohexyl)-para-cresol
M=421
CAS=77-62-3
2,4-Dimethyl-6-(1-methylcyclohexyl)phenol
M=218
CAS=77-61-2
N,Nxe2x80x2-Hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide)
M=637
CAS=23128-74-7
Triethylene glycol bis(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate
M=587
CAS=36443-68-2
and also sterically hindered polymeric phenols. One example of such products is a polymer having a molecular weight of between 700 and 800 and the CAS number 68610-51-5, which is sold under the trademark Lowinox(copyright) CPL by the company Great Lakes.
Of these products, it is advantageous to choose the sterically hindered phenol of the following formula (I): 
This product has the CAS number 2 27 676-62-6 and possesses a molecular mass of 784. It comprises a hindered phenol which may be also be regarded as a mixture of isocyanurate and triazines with alkylphenol substituents.
The compositions of the invention advantageously contain the sterically hindered phenol in an amount of from 0.05 to 0.4% by weight, based on the solids content of the composition.
As already seen above, the invention relates to all photopolymerizable, unsaturated acrylic varnishes, especially all UV-photopolymerizable, unsaturated acrylic varnishes.
As unsaturated acrylic derivatives, which form an essential constituent of these compositions, varnishes of this kind generally comprise acrylic derivatives selected from the unsaturated acrylic derivatives having a urethane, polyester or acrylate structure. These varnishes are advantageously composed substantially of the following constituents:
Polyol Esters Such as:
ditrimethylolpropane tetra- or triacrylate,
trimethylolpropane triacrylate,
pentaerythritol tetra- or triacrylate,
pentaerythritol alkoxy tetra- or triacrylate,
dipentaerythritol penta- or hexaacrylate,
acrylic polyester polyols in general.
Polyurethane Acrylates Such as:
Derivatives of polyols modified with aliphatic isocyanates such as IPDI, HDI, HMDI, with hydroxyethyl acrylate or hydroxybutyl acrylate termination.
Acrylic Acrylates Such as:
acrylic prepolymers whose hydroxyl groups are esterified with acrylic acid.
Furthermore, they additionally comprise the additives which are customary in compositions for the preparation of acrylic varnishes.
The compositions of the invention therefore advantageously comprise surface-active additives such as polysiloxane derivatives, monofunctional acrylic monomers for improving the varnish adhesion, adhesion promoters, especially in the form of carboxylated oligomers or methacryloylphosphoric acid derivatives, and photoinitiators for triggering polymerization under a UV lamp.
Furtherore, besides the sterically hindered phenol, the compositions of the invention advantageously comprise other stabilizers as well.
In particular, they further comprise at least one UV absorber from the class of the triazines, preferably a triazine having one of the following structures: 
A mixture of the products of the above formulae (II) and (IIxe2x80x2) is available commercially under the trademark Tinuvin(copyright) 400.
The product of the formula (III) is available under the trademark Cyagard(copyright) UV-1164L, the C18H17 group comprising a mixture of isomeric isooctyl groups.
Moreover, the varnish compositions of the invention further comprise at least one UV absorber from the class of the benzotriazoles. Of these, mention may be made in particular of 2-(2xe2x80x2-hydroxy-5-methylphenyl)benzotriazole having a molecular weight of 225 and the CAS number 2440-22-4.
Examples that may be mentioned of commercially customary products which are especially suitable as benzotriazoles for the acrylic varnish compositions of the invention are the commercial products with the trademark Lowilite(copyright) 55 or Tinuvin(copyright) P.
In the compositions of the invention, these benzotriazoles are present preferably in a concentration of between 0.2 and 0.5% by weight, based on the solids content of the composition.
In a particularly advantageous embodiment, the coating compositions of the invention comprise simultaneously a sterically hindered phenol, at least one triazine, and at least one benzotriazole, in the proportions indicated above.
In the choice of the proportions of these various constituents, account is taken of the fact that the triazine may give rise to bleeding or hazing, as a result of incompatibility with the polymer matrix after polymerization and aging under wet heat conditions, and that the benzotriazole derivatives alone may inhibit the polymerization reaction, since they absorb at wavelengths between 360 and 390 nanometers. These triazole derivatives are therefore used advantageously in relatively small amounts and as a supplement to an absorber of the triazine type.
As already mentioned above, the invention provides a possibility of stabilizing any acrylic varnishes, including both elastic and hard acrylic varnishes. The invention is particularly advantageous, however, in the case of compositions for hard acrylic varnishes in which the presence of sterically hindered phenol makes it possible to remedy the disadvantage that HALS compounds cannot be used to improve the transparency in the compositions without adversely affecting the varnish adhesion properties.
In accordance with one of its other substantial features, therefore, the invention relates to the use of a sterically hindered phenol as defined above as a stabilizer for a photopolymerizable acrylic varnish for the purpose of preventing the loss of adhesion of the varnish on a substrate.
The invention is especially suitable for the adhesion of acrylic varnishes to polycarbonate or PMMA substrates. As already evident from the above remarks, it is suitable in a particularly advantageous manner for the application of a hard acrylic varnish, i.e., an acrylic varnish having a Persoz hardness in accordance with NFT standard 300 16 of more than 300 seconds.
The example which follows is intended to illustrate the invention without restricting it.